Normal bone growth and remodeling are, in part, dependent upon factors that influence the number and activity of osteoclasts. Skeletal pathologies may result from alterations in the osteoclast plasma membrane or in signals that govern osteoclast cytodifferentiation and resorptive activity. Despite substantial recent insights into understanding the hematopoietic lineage and the interactions of local and systemic modulators of osteoclast development and function, many questions remain. In addition, the biochemical and molecular profile of the mature osteoclast is incomplete. The hypothesis upon which this application is based proposes that osteoclast development entails specific cell surface alterations and that discrete membrane-associated molecules are involved in osteoclast regulation and function. The specific aims of this proposal are to: 1) further characterize the antigen reactive with the 121F monoclonal antibody (MAb) and to clarify its relationship with superoxide dismutase, 2) continue to identify and biochemically describe distinct osteoclast plasma membrane molecules recognized by a panel of osteoclast-specific MAbs, 3) investigate cell surface changes associated with osteoclast development and functional state, relative to the effects of known modulators of osteoclast development and activity (such as estradiol, calcitonin, interleukins 1 and 6, and TNF) and to the induced expression of protooncogenes (c-fos, c-jun, and c-src), and 4) identify osteoclast precursors and define stages of osteoclast cytodifferentiation using-these cell surface markers and molecular probes. Highly purified chick osteoclasts and a human leukemic cell line (FLG 29.1) will be used as sources for chicken and human antigens. In other experiments, marrow mononuclear cells which fuse to form multinucleated osteoclast-like giant cells in culture will serve as a developmental system in which to investigate the influence of known bone modulators on antigen expression and resorptive capability, and to define osteoclast precursors. Biochemical,physiological, immunohistochemical, and molecular cloning techniques will be employed to ascertain the nature and functional role of these antigens. Western and northern blot analyses will be used to examine their relationship to protooncogene expression. These results should not only expand our basic understanding of the cellular and molecular biology of the osteoclast, but also potentially aid in devising appropriate preventative and therapeutic strategies for the skeletal osteopenia of arthritis, osteoporosis, periodontal disease, and other local or metabolic disorders of bone.